1. Field of the Invention
Embodiments of the invention relate to the field of plasma processing. More particularly, the present invention relates to a method and system for monitoring plasmas.
2. Discussion of Related Art
In plasma-based processing tools, power is input through a series of hardware components that couple the power into a plasma. When the plasma is ignited a large fraction of the input power may be dissipated into the plasma, which serves as a load. However, when the plasma is extinguished, applied power may be dissipated in hardware components and may damage components, such as dielectric windows, matching network, and generator. It may therefore be desirable to monitor a plasma to minimize the time during which a plasma is absent while power is being generated.
In many known plasma systems, detectors, such as optical detectors, are used to aid in monitoring the plasma in a plasma processing chamber. Such detectors are typically located external to the plasma chamber and may be mounted adjacent to transparent windows through which optical signals to and from the plasma can propagate. However, in the case of plasma processes that involve material deposition, the optical window may become opaque after a period of material deposition, causing the optical detection system to fail to properly detect the presence or absence of a plasma.
FIG. 1 is a schematic illustration of a known plasma system 10 that may be used for ion implantation. As depicted, plasma system 10 is arranged as a plasma doping (PLAD) system that comprises a process chamber 12 having a pedestal or platen 14 to support an insulated target substrate 5. One or more reactive gases containing the desired dopant characteristics are fed into the process chamber 12 via a gas inlet 13 through a top plate 18 of the chamber. The reactive gas may be, for example, BF3, B2H6, PH3, etc. The reactive gas(es) may then be distributed uniformly via baffle 11 before entering the process chamber 12. A group of coils 16 which together with the walls of chamber 12 form an anode may couple radio frequency (RF) electrical power into the process chamber 12 through an aluminum oxide (Al2O3) window 17. The RF power produces a dopant-containing plasma 20 from the reactive gas(es). A bias voltage is applied to the target substrate 5 via the platen 14 to draw charged particles from the plasma 20. The platen 14 is electrically insulated from chamber walls of system 10 and the target substrate is kept at a negative potential to attract the positively charged ions of the plasma. Typically, the substrate 5 is biased with a pulsed DC voltage to act as a cathode. As a result, dopant ions are extracted from the plasma 20 across a plasma sheath disposed between plasma 20 and a top surface of substrate 5. The ions are implanted into the substrate 5 during the bias pulse-on periods. Generally, an ion dose is the amount of ions implanted into the target substrate or the integral over time of the ion current. The bias voltage corresponds to the implantation depth of the ions which may also be influenced by the pressure and flow rate of the reactive gas introduced into process chamber 12, duration of the bias voltage, etc. System 10 also includes an optical monitor 28, which may be used to monitor a plasma during processing.
Because plasma system may employ reactive gases such as BF3, B2H6, PH3, etc., during operation of implantation system 10, products of such gases may condense on walls of process chamber 12. With sufficient time, such condensates may form opaque films on surfaces including any optically transparent windows that otherwise facilitate monitoring of the plasma 20. In this manner, any initially transparent windows of process chamber 12 may become substantially opaque during operation, such that visible radiation from plasma is not transmitted outside the chamber, thereby preventing optical monitoring of the plasma using optical monitor 28. Accordingly, any anomalies, such as the failure of a plasma to ignite when subject to RF power, may be undetectable. Moreover, if optical monitoring is used to control a plasma process, the accumulation of opaque material on an initially transparent window may reduce the light signal from a plasma to the point where the optical monitor 28 determines that a plasma is no longer present. Even though a plasma is actually present, the optical detector may inadvertently trigger a shutdown of source power if system controls are set to power down when a plasma extinguishes. In view of the above, it will be appreciated that there is a need to improve monitoring of plasma processes, including those that involve material deposition.